Evidence is accumulating that moderate levels of sustained local hyperthermia, alone or in conjunction with other methods of therapy, is useful in the management of malignant tumors. A scaled-down laboratory prototype of a computer-controlled insonation system for deposition of ultrasonic energy within a predetermined volume of tissue, by scanning the region in a programmed manner, has been demonstrated to produce and maintain the prescribed level of hyperthermia throughout the target volume for the desired duration. No evidence of morphological tissue damage was observed histologically in specimens with temperature elevation to 42.5 degrees C sustained for periods of up to 6 hours. Design and testing of a full-scale clinical model is in progress.